1. Field of the Invention
This invention relates to an automatic gap adjusting device for a disc brake incorporating therein a mechanical brake mechanism.
2. Description of the Prior Art
In the conventional device of this type, as shown in FIG. 1 of the accompanying drawings, a spindle 5 which can be moved forward to a disc rotor 2 by an external force exerted by a parking lever or the like is disposed within a cylinder 4 containing a hydraulic operating piston 3 which is arranged to push a friction pad 1 against the rotor 2 for brake application. During service brake application which is effected by hydraulic pressure, only the piston 3 is allowed to move forward toward the rotor 2 while the spindle 5 is kept stationary. The wear of the lining of the friction pad 1 is taken as increase in the extent of relative displacement between the piston 3 and the spindle 5 to use it for redetermining the returned reposing position of the piston 3 (a gap adjusting action) and, at the time of parking brake application, an external force is exerted to move the spindle 5 together with the piston 3. The conventional gap adjusting mechanism is arranged as follows: A sleeve nut 6 is screwed to a multiple thread screw part 5a of the fore end of the spindle 5. When the extent of the forward movement stroke of the piston 3 caused by hydraulic pressure during service brake application comes to exceed the extent of the movable play (corresponding to a gap between the friction pad 1 and the disc rotor 2) provided for allowing a bearing 8 interposed in between the sleeve nut 6 and a stopper screwed to the piston 3 to move in the axial direction of the piston 3, the sleeve nut 6 is arranged to rotate on the screw to follow the piston 3. The inward screwing rotation of the sleeve nut 6 is arranged to be restricted by a clutch spring 9 which has its winding part 9a wound round the sleeve nut 6 and its one end inserted in a blind hole 3a provided in an inner face of the piston 3 in the axial direction of the piston. The sleeve nut 6 is thus arranged to screw outwardly according as the extent of the forward movement stroke of the piston 3 increases as a result of the wear of the lining of the friction pad 1. The returned reposing position of the piston 3 is thus gradually redetermined relative to the disc rotor 2 with the fore end of the sleeve nut 6 allowed to abut upon an inner face of the piston 3.
With the conventional device arranged as described in the foregoing, when the sleeve nut 6 tilts relative to the axis of the piston 3, the above stated extent of movable play provided for the bearing 8 in the axial direction of the piston becomes less than a setting value thereof. With the movable play of the bearing 8 decreased in this manner, the returning extent of the piston 3 to a brake releasing position becomes less than a normal extent. Such an insufficient returning movement of the piston 3 has been causing the trouble of so-called "dragging" with the friction pad 1 remaining in contact with the disc rotor 2 under a brake releasing condition. Further, when the sleeve nut 6 tilts, a contact area between the piston 3 and the sleeve nut 6 decreases and a frictional force between them becomes insufficient. Then, during parking brake application under such a condition, an excessive force is exerted on the clutch spring 9. This necessitates the clutch spring 9 to have a great strength.